The present invention pertains to hot-melt adhesives for bonding vitreous plastic materials to thermoplastic or thermosetting plastic materials, as well as their use for bonding lenses to lamp bodies of vehicle lamps or headlamps, as well as the vehicle lamps or headlamps prepared using said hot-melt adhesives.
Hot-melt adhesives belong to the physically setting type of adhesive which mostly solidify without chemical cross-linking or evaporation of solvents, only due to a congealing process with decreasing temperature. Reactive hot-melt adhesives set after the mounting and achieve their final strength through a subsequent chemical reaction (cross-linking).
Hot-melt adhesives have a number of advantages over chemically reacting and solvent-containing adhesives:
absence of solvents or monomers and thus little environmental and industrial safety requirements;
dosage and mixing errors by machines or the staff are not possible;
very short setting times of a few seconds to about one minute; and
thermally undoable assembly.
However, some fundamental disadvantages also result therefrom:
tendency to creeping, mainly at elevated temperature and permanent static load;
heat load of the assembly members in the application of the hot-melt adhesives;
relatively low level of temperature resistance as compared to chemically cross-linking systems; and
necessity of providing appropriate melting plants, the working viskosities of hot-melt adhesives being essentially higher, as a rule, than those of two-component or solvent-containing adhesive systems.
Hot-melt adhesives known in the prior art are predominantly made of the materials polyamide, saturated polyesters or polyurethanes, and ethylene-vinyl acetate copolymers. In addition, non-polar butyl sealant compositions are frequently found in sealing and mounting technologies. These systems are generally composed of tackifying resins, backbone polymers, flexibilizers, fillers, stabilizers and adhesion promoters.
The melting behavior is usually not sharply limited to a narrow range of temperatures, but comprises a melting range over a temperature difference of 20 to 100xc2x0 C. This is also the amount by which the maximum temperature of use is at least different from the minimum application temperature.
In order to achieve special desired properties, especially suitable components having quite different melting and/or softening ranges, different molecular weights and in part critical compatibilities must be blended together to give as homogeneous as possible a hot-melt adhesive.
Since the polymers used in the prior art also in part have molecular weights of around 106 g/mol, high shearing forces must be active during the blending. In order to treat the higher molecular weight polymers gently in this process and generally to accelerate dissolution processes, the materials, as is well-known, are incorporated in one another at elevated temperature in a suitable mixing apparatus. This may be, for example, a twin-screw kneader in which the hot-melt adhesive is blended at about 100 to 200xc2x0 C., usually 120 to 160xc2x0 C. As an example of the very extensive prior art, reference is made to U.S. Pat. No. 4,857,594-A which describes a hot-melt adhesive which is particularly suitable for bonding polyolefin moldings to identical or other polyolefin moldings as well as other, non-polyolefin moldings.
This hot-melt adhesive consists of
(A) from 60 to 100% by weight of a mixture consisting of an amorphous polypropylene (xcex1-polyolefin), a block copolymer selected from the group consisting of styrene-ethylene/butene-1 block copolymers and styrene-ethylene/propylene block copolymers, and a tackifying hydrocarbon resin wherein said amorphous polypropylene has a crystallinity of less than 10% by weight and a molecular weight in the range of from 300 to 50 000, and said hydrocarbon resin has a ring and ball softening point in the range of about 10xc2x0 C. to 150xc2x0 C., the components being present in the following relative amounts:
amorphous polypropylene from 5 to 95% by weight
block copolymer from 2.5 to 40% by weight; and
hydrocarbon resin from 2.5 to 55% by weight; and
(B) from 0 to 5% by weight, based on the weight of the finished hot-melt adhesive, of an antioxidant; and
(C) from 0 to 40% by weight, based on the weight of the finished adhesive composition, of a compound or mixture of compounds selected from the group consisting of an isotactic polypropylene, ethylene-propylene copolymers, paraffin and microcrystalline waxes, Fischer-Tropsch waxes, low-molecular weight polybutenes, polyisobutylene, low-molecular weight polyethylenes, low density polyethylene, and process oils.
Due to the kind of substrates to be bonded together, i.e. essentially thermoplastic polyolefins, the range of materials to be used is wide. In special applications, however, the basic formulations are too expensive and usually unsuited for permanently bonding, for example, vitreous surfaces, such as polycarbonate.
U.S. Pat. No. 4,783,504-A describes a solvent-free hot-melt adhesive which is said to be suitable, inter alia, for sealing insulating glass or vehicle headlamps with glass lenses. The composition comprises (a) 100 parts by weight of a silane-functionalized polymer component, and (b) a certain amount of a medium block component compatible therewith for controlling the glass transition temperature. Said silane-functionalized polymer is employed in order to achieve adhesion to glass.
Lenses made of vitreous plastic materials are connected to lamp bodies in the prior art usually with sealant materials and mechanical fixation by clamps. First, the lens is provided with a scratchproofing coating, especially by spraying and subsequent curing under UV irradiation.
The sealant materials proposed in the prior art for sealing the space between the lamp body and the plastic lens required a mechanical fixation of the lens, for example, with clamps, in order to ensure a permanent custom-fit positioning during the use of the vehicle.
Accordingly, it has been the object of the present invention to provide, in particular inexpensive, hot-melt adhesives for bonding vitreous plastic materials to thermoplastic or thermosetting plastic materials.
In a first embodiment, the above-mentioned object is achieved by hot-melt adhesives for bonding vitreous plastic materials to thermoplastic or thermosetting plastic materials, containing:
a) from 0.5 to 15% by weight, in particular from 0.5 to 10% by weight, of optionally grafted thermoplastic elastomers;
b) from 5 to 40% by weight, in particular from 10 to 35% by weight, of optionally grafted xcex1-polyolefins;
c) from 5 to 45% by weight, in particular from 15 to 40% by weight, of adhesive resins; and
d) from 5 to 55% by weight, in particular from 15 to 45% by weight, of polyisobutylenes.
The hot-melt adhesives according to the invention can be manufactured in cartridges or barrel-like containers or as an open bulk material. When cartridges are used, the whole adhesive is frequently heated and processed by means of a manual applicator. In the case of larger barrel-like containers, a heated die having a removal and conveying line is brought on the surface of the hot-melt adhesive in a stationary plant, and the adhesive is removed only from the surface region. This involves heating of only part of the hot-melt adhesive. Since the hot-melt adhesive is forcibly conveyed, there are no high requirements to its flow characteristics in this kind of application technique.
On the other hand, this cumbersome provision of the hot-melt adhesive can be dispensed with by employing an open bulk material, optionally with adhesive paper as a separating layer, using tank melting plants. In this case, blocks of hot-melt adhesive are added into a heated tank, and the melt is supplied to the application nozzle via a pump, entering through the removal opening positioned at the bottom.
In this way of providing the hot-melt adhesive melt for conveying, dosage and application to the substrates, higher requirements are demanded of the conveying and flowing characteristics of the hot-melt adhesive. As compared with conventional hot-melt adhesives, the conveyability of the hot-melt adhesives according to the invention is of particular importance.
The flowability of the hot-melt adhesive is determined, in particular, by the choice of high-molecular weight fractions. If, for example, only low-molecular weight fractions and minute amounts of high-molecular weight materials are employed, a sufficient cohesive strength cannot be achieved.
In addition, significant differences with respect to the prior art can be seen in the reversibility of the hot-melt adhesives according to the invention. In addition to the setting of hot-melt adhesives, which is mainly based on physical effects, it is also possible, evidently, to combine these effects with an additional chemical cross-linking.
Replacing thermoplastic elastomers by cross-linked butyl rubber results in a lesser reversibility, although a well-flowable hot-melt adhesive can also be realized using such polymers.
Especially mechanism of failure, i.e. cohesive or adhesive failure, is of particular importance according to the invention. By means of the present invention, a cohesive failure mechanism is obtained with maximum amounts of tensile strength and crack resistance. For cold flexibility, high polyisobutylene contents are required. According to the invention, it is shown that the excellent values of adhesion to all substrates to be bonded in mutual combination of the raw materials could be achieved, in particular, by the modification with hydrocarbon resins. Good adhesion to thermoplastic materials is achieved mainly by xcex1-polyolefins, especially maleic anhydride grafted xcex1-polyolefins. The main important fact is that the substrates can be assembled as-sprayed using the hot-melt adhesives according to the invention without a special additional treatment which is to enable adhesion of the adhesives to the substrates.